Gaseous oxygen produced by air separation plants are usually at elevated pressure about 20 to 50 bar. The basic distillation scheme is usually a double column process producing oxygen at the bottom of the low pressure column operated at 1.4 to 4 bar. The oxygen must be compressed to higher pressure either by oxygen compressor or by the liquid pumped process. Because of the safety issues associated with the oxygen compressors, most recent oxygen plants are based on the liquid pumped process. In order to vaporize liquid oxygen at elevated pressure there is a need for an additional motor-driven booster compressor to raise a portion of the feed air or nitrogen to higher pressure in the range of 40–80 bars. In essence, the booster replaces the oxygen compressor.
In the effort to reduce the complexity of an oxygen plant, it is desirable to reduce the number of motor-driven compressors. Significant cost reduction can be achieved if the booster can be eliminated without much affecting the plant performance in terms of power consumption. Furthermore, the air purification unit conceived for a traditional oxygen plant would operate at about 5–7 bar which is essentially the pressure of the high pressure column, and it is also desirable to raise this pressure to a higher level in order to render the equipment more compact and less costly.
A cold compression process as described in U.S. Pat. No. 5,475,980 provides a technique to drive the oxygen plant with a single air compressor. In this process air to be distilled is chilled in the main exchanger then further compressed by a booster compressor driven by an expander exhausting into the high pressure column of a double column process. By doing so, the discharge pressure of the air compressor is in the range of 15 bar which is also quite advantageous for the purification unit. One inconvenience of this approach is the increase of the size of the main exchanger due to additional flow recycling which is typical for the cold compression plant. One can reduce the size of the exchanger by opening up the temperature approaches of the exchanger. However, this would lead to inefficient power usage and higher discharge pressure of the compressor and therefore increasing its cost. An illustration of this prior art is presented in FIG. 1, in which an oil brake is added to the system to dissipate the power required for the refrigeration. In larger plants, an expander can replace the oil brake.
In FIG. 1 all the feed air is compressed in compressor 1, purified in purification unit 2 and sent as stream 11 to the warm end of the heat exchanger 5. All the feed air is cooled to an intermediate temperature, removed from the heat exchanger as stream 7 and compressed in cold compressor 8. The compressed stream 9 is sent back to the heat exchanger at a higher intermediate temperature, cooled to a temperature lower than the inlet temperature of the cold compressor 8 and divided in two. Stream 15 is sent to the Claude expander 13 which is braked by the compressor 8 and an oil brake. The rest of the air 10 is liquefied in the heat exchanger and divided into two parts, one part being sent to the high pressure column 30 and the rest 34 being sent to the low pressure column 31.
An oxygen enriched liquid stream 28 is expanded and sent from the high pressure column to the low pressure column. A nitrogen enriched liquid stream 29 is expanded and sent from the high pressure column to the low pressure column. High pressure gaseous nitrogen 14 is removed from the top of the high pressure column and warmed in the heat exchanger to form a product stream 24. Liquid oxygen 20 is removed from the bottom of the low pressure column 31, pressurized by a pump 21 and sent as stream 22 to the heat exchanger 5 where it vaporizes by heat exchange with the pressurized air 10 to form gaseous pressurized oxygen 23. A top nitrogen enriched gaseous stream 25 is removed from the low pressure column 31, warmed in the heat exchanger 5 and then forms stream 26.
Some different versions of the cold compression process were also described in prior art as in U.S. Pat. Nos. 5,379,598, 5,596,885, 5,901,576 and 6,626,008.
In U.S. Pat. No. 5,379,598 a fraction of feed air is further compressed by a booster compressor followed by a cold compressor to yield a pressurized stream needed for the vaporization of oxygen. This approach still has at least two compressors and the purification unit still operates at low pressure.
In U.S. Pat. No. 5,596,885, a fraction of the feed air is further compressed in a warm booster whilst at least part of the air is further compressed in a cold booster. Air from both boosters is liquefied and part of the cold compressed air is expanded in a Claude expander.
U.S. Pat. No. 5,901,576 describes several arrangements of cold compression schemes utilizing the expansion of vaporized rich liquid of the bottom of the high pressure column, or the expansion of high pressure nitrogen to drive the cold compressor. In some cases, motor driven cold compressors were also used. These processes also operate with feed air at about the high pressure column's pressure and in most cases a booster compressor is also needed.
U.S. Pat. No. 6,626,008 describes a heat pump cycle utilizing a cold compressor to improve the distillation process for the production of low purity oxygen for a double vaporizer oxygen process. Low air pressure and a booster compressor are also typical for this kind of process.
Therefore it is a purpose of this invention to resolve the inconveniences of the traditional process by providing a solution to simplify the compression train and to reduce the size of the purification unit. This can moreover be achieved with good power consumption. The overall product cost of an oxygen plant can therefore be reduced.